1. Field of the Invention
This invention resides in the technology of recycling waste rubber, particularly from used vehicle tires.
2. Background of the Invention
The Rubber Manufacturers Association reports that approximately two billion used tires have already been discarded in the United States and that the number discarded each year is approximately 250 million. Uncontrolled incineration is not an acceptable means of disposal of tire rubber, and burial in landfills is increasingly difficult as the availability of landfills diminishes.
To avoid the problems of disposal, various means have been developed to extract usable rubber from used tires. Reclamation processes generally begin with the comminution of the tire material into one-inch to two-inch tire chips, which are then used as an energy source in the generation of electricity and the production of paper and, cement. Tire chips that are ground to micron sizes are termed xe2x80x9ccrumb rubber.xe2x80x9d Crumb rubber is mixed with various polyurethanes, surface modifiers, polymers derived from maleic anhydride, or combinations of these additives in the manufacture of roofing materials, walk pads, carpet backings and pads, and flooring underlays.
Forming useful objects from crumb rubber generally requires the addition of various fillers and resins to the crumb rubber to adhere the particles together and to adjust the physical characteristics such as hardness, dimensional stability and wear resistance of the product. Examples of the types of fillers and resins used in preparing construction materials from crumb rubber are found in Crivelli et al., U.S. Pat. No. 5,258,222 (Nov. 2, 1993), which describes the incorporation of coarse siliceous grains and a polymerizable liquid binder. Aqueous dispersions of neoprene and other elastomeric dienes or of an epoxide monomer are cited as examples of suitable binders. These additives, and especially the binders, add considerably to the cost of the formulation, and by relying on the additives to control the properties of the finished product, this adds even further to the cost and complexity of the formulation and diminishes the amount of recycled rubber that can be used. Similar formulations are disclosed by several patents to Kiser (Environrmental, L.L.C.), U.S. Pat. No. 5,453,313 (Sep. 26, 1995), U.S. Pat. No. 5,525,399 (Jun. 11, 1996), U.S. Pat. No. 5,582,864 (Dec. 10, 1996), and U.S. Pat. No. 5,587,234 (Dec. 24, 1996). The binder in these formulations is an elastomeric polysulfide, which must comprise 50% or more by volume of the formulation. A still further formulation is that of Lanphier, U.S. Pat. No. 5,527,409 (Jun. 18, 1996), which includes a urethane binder and a compaction process to form the comminuted tire particles into a log, from which a sheet is then cut.
In addition to the expense of including these binders and the limitation that the binders place on the amount of tire rubber that can be included, the highly granular nature of the formulations limits the range of techniques that can be used to form them into products of the desired shape. In particular, the formulations in these reference patents are not suitable for injection or calender molding.
It has now been discovered that a formulation useful for preparing roofing materials and other rubber-based articles in a manner that provides a high degree of control over stiffness and other characteristics of the articles without the need for binders, is formed by:
(a) devulcanizing crumb rubber by conventional devulcanization methods to achieve a devulcanized material with a plasticity of less than 120 Mooney units, and preferably less than 80 Mooney units, and whose rheometer cure curve has a TS2 value of greater than thirty seconds;
(b) formulating the devulcanized rubber with sulfur, a compound containing bound water, and a flame retardant.
The resulting formulation can be molded in various ways, including compression molding, extrusion molding, injection molding, and calender molding, and can be made with a rubber content exceeding 50% by weight (of the crumb rubber used in the formulation). In addition to being economical, the molded product is highly versatile in use, highly fire resistant, heat insulative and impermeable to moisture, and achieves these qualities at minimal cost. These qualities are further achievable without the inclusion of such additives as oxidants, heavy metals, or halogen-containing rubber additives. A quality of the molded product that makes it particularly useful for roofing materials is that upon ignition, the product will self-extinguish in less than ten seconds, generally less than five seconds, and often within zero to two seconds.
Further details of these and other features and advantages of the invention will be more fully understood from the description that follows.